Optical modulators are used to electrically modulate a laser beam from a laser source for optical communication, scientific instruments and other instruments. One type of optical modulator includes a crystal (EO material) and a drive circuit that applies a driving voltage across the crystal. In this design, the laser beam is directed at the crystal. The voltage across the crystal changes the index of refraction of the crystal. The amount in which the index of refraction is changed is proportional to the amount of voltage applied to it. With this design, the crystal changes the path length of the laser beam through the crystal in accordance with the frequency of the driving voltage. Thus, a laser beam emerging from the crystal is modulated by the frequency of the driving voltage.
Designers are always trying to increase the efficiency of the modulator, i.e. reduce the driving voltage or power requirement of the optical modulator. Efficiency of the optical modulator is fundamentally determined by the EO materials used in optical modulator. However, one way to increase effective efficiency is to exploit the dielectric nature of EO material. For example, the EO material and the conductive electrode connected to the EO material forms a capacitor. By connecting the capacitor with other circuit elements such as inductors, or by placing the EO material in a resonant cavity, one can increase the charge or electrical field across capacitor by accumulator charges through resonant effect. The figure of merit of resonant circuits is its Q. The efficiency of resonant optical modulators is proportional to the square root of cavity Q. High Q cavity design requires a conductive cavity (metals, such as Al, Au, etc) in close contact with the dielectric EO material.